Neurofibromatosis type 1 (NF1) is a genetic disorder caused by mutations in the NF1 tumor suppressor gene. Children with NF1 are predisposed to developing juvenile myelomonocytic leukemia (JMML). Neurofibromin, the protein product of NF1, is a negative regulator of p21ras activity. Though Nf1 -/- mice die in utero, mice reconstituted with Nf1 -/- fetal stem cells develop a myeloproliferative disease (MPD) similar to JMML in NF1 patients. However, alterations in p21ras signaling pathways in NF1 deficient cells responsible for MPD are unknown. Utilizing PI-3 kinase (PI3K) inhibitors, we have preliminary data implicating hyperactivation of the p21ras-PI3K pathway as responsible for the hyperproliferation and increased survival of Nf1 -/- cells. However, interpretation of results using PI3K inhibitors is limited because there are four classes of PI3K, and inhibitors inactivate all classes. Here we propose genetic experiments to determine whether hyperactivation of class IAPI3K alters the growth of Nf1 -/- hematopoietic cells. Our rationale for studying class IAPI3K in Nf1-/- cells is twofold. First, in contrast to other PI3Ks, all class IAPI3K catalytic subunits contain a p21ras-binding domain. Second, p21ras interacts with these subunits to augment kinase activity in vitro, and no evidence exists to show that p21ras augments the activity of other PI3K classes. Recently, a p85alpha (a regulatory subunit of class IAPI3K) knockout strain was generated which results in a 97% reduction in class IAPI3K activity in myeloid cells. We hypothesize that hyperactivation of the p21ras-class IAPI3K pathway, alters the proliferation and survival of Nf1 -/- hematopoietic cells and contributes to the progression of MPD in mice transplanted with Nf1 -/- cells. To test this hypothesis, we will conduct experiments utilizing a genetic intercross of Nf1 +/- and p85alpha knockout mice. The aims are: 1) To test whether hyperactivation of class IAPI3K contributes to MPD in mice reconstituted with Nf1 -/- stem cells by altering specific signaling pathways, 2) To examine how genetic inactivation of class IAPI3K alters the proliferation and survival of committed, multipotential, and primitive Nf1 +/+ and Nf1 -/- progenitor cells, 3) To examine how neurofibromin and class IAPI3K regulate cell cycle progression and survival of phenotypically defined hematopoietic cells in vivo.